An Inertial Reference System (IRS) platform needs to be leveled and aligned upon power up before it can be used for navigation. Leveling refers to the process of estimating the initial roll and pitch angle of the platform with respect to the local vertical, while alignment refers to the process of estimating the platform heading angle. It is desirable that the platform is stationary during leveling and alignment.
This is typically not a problem for commercial fixed-wing aircrafts. However, for helicopters, the IRS platform motion can be significant (e.g., if the rotors are on, or if the helicopter is being deployed from an aircraft carrier, or if the helicopter is power cycled in air). The movement of the helicopter degrades the accuracy of the estimated roll and pitch angle output from the leveling algorithm.
Although leveling and alignment can be done while the helicopter is in motion, the algorithms used when in motion are different and the completion time is significantly increased.